The Night Mother's coffin must be first unlocked from the outside before you can slip in. Once opened, head inside until the screen turns dark. (Not creepy at all being in a coffin with a dessicated corpse.) It's just a matter of being patient while Cicero spews his madness and then popping out to surprise Cicero afterward.

It is worth noting that before performing this quest, the player can find out that he or she is the listener by shouting around Cicero (or at him, if desired). He will comment with, "For someone called the Listener, you do an awful lot of shouting," thus indicating the player's role as Listener ahead of time.